The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
In computer networks such as the Internet, packets of data are sent from a source to a destination via a network of elements including links (communication paths such as telephone or optical lines) and nodes (for example, routers directing the packet along one or more of a plurality of links connected to it) according to one of various routing protocols.
One such protocol is MPLS (Multi Protocol Label Switching), which is described in the document “Multi Protocol Label Switching Architecture” which is available at the time of writing on the file “rfc3031.txt” in the directory “rfc” of the domain “ietf.org” on the World Wide Web. According to MPLS, a complete path for a source-destination pair is established, and values required for forwarding a packet between adjacent label switched routers (LSRs) in the path together with headers, or tags or “labels” are pre-pended to the packet. The labels are used to direct the packet to the correct interface and next hop. The labels precede the Internet Protocol (IP) header or other header allowing smaller outer headers.
The path for the source-destination pair, termed a Label Switched Path (LSP), can be established according to various different approaches. One such approach is the Label Distribution Protocol (LDP) in which each router in the path invokes an LDP session with neighboring LSRs and sends its label to the next hop router on the path as determined from its IP routing table. Alternative label distribution mechanisms include Resource Reservation Protocol (RSVP) in which case, for example, a network administrator can engineer a path, providing strict source routing and modifications to Interior Gateway Protocols (IGPs). In all cases a Label Forwarding Information Base (LFIB) stores both the next-hop information for the LSP, together with the label required by the next hop as a label binding.
For each LSP created, a forwarding equivalent class (FEC) is associated with the path specifying which packets are mapped to it. At an ingress LSR to the LSP, packets destined, for example, for a certain destination or “prefix” are assigned to a corresponding FEC and injected into the LSP with the LSP next-hops ingress label pre-pended. The LSP next-hop router swaps its ingress label with an egress label received from its next-hop router and so forth. At an LSP egress router, the ingress label is removed and the packet is forwarded on towards the destination prefix according to the routing protocol supported thereafter.
One known use of MPLS is in MPLS virtual private networks (VPNs), where an LSP is established between ingress and egress provider edge routers (PE) accessible by respective customer edge (CE) routers, providing an effective tunnel between customer edge routers.